1. Field of the Invention
This invention relates generally to exhaust manifolds for internal combustion engines, and more particularly to extending an exhaust manifold conduit into a downstream exhaust pipe.
2. Description of the Related Art
In a "four-into-two-into-one" exhaust system for a four-cylinder engine, the primary exhaust manifold conduits from a first pair of cylinders communicate into a separate secondary exhaust pipe, and the primary exhaust manifold conduits from a second pair of cylinders communicate into another separate secondary exhaust pipe. The two secondary exhaust pipes are joined together into a single downstream tailpipe.
When a pair of exhaust manifold conduits delivers two streams of exhaust gas into the mouth of a downstream exhaust pipe, from different directions, turbulence is created at the mouth. Eddies, which are currents of exhaust gas running contrary to the main current, tend to build up along the wall of the downstream exhaust pipe where friction impedes exhaust gas flow. The eddies hinder movement of exhaust gases from the upstream cylinders. Turbulence and eddies in the flowing exhaust gas cause back pressure, which reduces engine performance.
Expansion of the exhaust gas occurs at each junction until final expansion at the open end of the tailpipe. Rarefaction waves return as reflections from the expansion of the exhaust gas at the junctions and tend to push the exhaust gas back into the cylinders.
The length of an exhaust conduit from the exhaust port of a cylinder to a junction at which the conduit merges with one or more other conduits is known as "the resonant length". The resonant length of the exhaust conduit determines, at least in part, the period of the rarefaction wave.
The rate of closing of a valve (or set of valves) in the exhaust port depends on the revolutions per minute ("RPM") of the engine. For a particular range of engine RPMs, the period of the rarefaction wave will not coincide with periodic valve closure in the exhaust port if the resonant length is not within a certain range of lengths.
If the period of the rarefaction wave does not coincide with the period of valve closure in the exhaust port for certain engine RPMs, the rarefaction wave will not impinge at the exhaust port when the valve is closed therein, and will tend to push exhaust gas back into the cylinder while the valve is not closed.
Increasing the resonant length affects the period of the rarefaction wave, and can cause the rarefaction wave to impinge at the exhaust port in synchronization with the closing of the valve therein. It is, however, often difficult to fit additional conduit length into an engine compartment. As such, there is currently a need for an exhaust system which eliminates or reduces these concerns.